A circuit of the type defined in the foregoing is disclosed in German Auslegeschrift No. 2,647,428. In the carrier supply unit described there each electrical apparatus is provided with a spare apparatus so that in the event of breakdown of one of the apparatuses a spare apparatus is immediately switched-on with the aid of a change-over circuit and a supervisory circuit. Both apparatuses are of an identical construction, so that in normal conditions there is no preference for one apparatus over the other. It is consequently not necessary to change-over again to the new apparatus after the defective apparatus has been replaced. This has the advantage that the disturbances in the system, mentioned in the foregoing, are reduced.
In the prior art circuit a delay circuit is provided between the change-over circuit and the supervisory circuit. This makes it possible to replace the defective apparatus by a new apparatus during the delay produced by the delay circuit. The supply voltage will not be connected to the new apparatus until after the said delay has elapsed, thus reducing the disturbances mentioned in the foregoing even further.